1. Field of the Invention
The present invention relates to a method and system for enhancing the overall picture contrast of digital video signals by using an automatically configurable mapping function that is automatically selected from a predetermined set of mapping functions based on set criteria.
2. Description of the Related Art
The need for video contrast enhancement may arise due to the picture contrast distortions caused by transmission limitations or the limitations of video capturing devices or media. Examples of video signals requiring contrast enhancement are video pictures that are too dark, too bright, too low in contrast or are over contrast.
Mapping functions are normally used to map the original luminance values for each respective pixel in a picture or video frame into new respective values. FIGS. 1 to 4 illustrate different examples of such mapping functions. In FIGS. 1 to 4, the horizontal axis represents the original luminance values of those pixels in a picture and the vertical axis represents the adjusted pixel luminance values using each mapping function based on each pixel's original rumination value. The mapped output picture generally improves the contrast of the picture and gives a better subjective quality.
In U.S. Pat. Nos. 5,383,168 and 6,259,472, the mapping functions generated are based on the normalized Cumulative Density Function (“CDF”) of the picture histogram. In U.S. Pat. No. 6,148,103, mapping functions are generated by comparing the histogram for the current picture being processed with the CDF of previously processed histograms corresponding to earlier field pictures. In U.S. Pat. No. 5,937,090, the original picture is first quantized and then the CDF of the quantized picture histogram is used to generate the mapping functions. In U.S. Pat. No. 5,546,134, some pre-determined types of mapping curves are used, and the average brightness level in the video input signal is used to decide the type of curve to be used. In U.S. Pat. No. 5,808,697, each picture is divided into blocks and the average luminance level of each block, determined over a series of frames or pictures is used to select an appropriate mapping function for that block, and an enhancer maps each pixel in that block according to both the selected mapping function for that block and the mapping functions selected for adjacent areas, and the mapping results are combined by a weighted-average processor.
It is known that mapping functions generated by the CDF of the picture histogram tend to over-enhance or over-correct the contrast, resulting in over-contrast pictures. Mapping functions that are generated by picture pre-processing, for example performing quantization or filtering, may involve complex processing, which can be slow to process.
As for the selection of predetermined types of mapping curves using only the average brightness of blocks in each picture, the technique may be too simplistic and lack the accuracy required for determining the most appropriate mapping function to be used. Methods that divide a picture into blocks and select mapping functions for each block according to its difference with the average block luminance level can be highly complex and flickering could also result.
For any existing system that has selectable or switchable mapping functions in place but lack an automatic mapping functions selection mechanism, the existing techniques described above either require too complex design changes to such a system or the mapping function selection method is too simplistic and lacks accuracy.
A need therefore exists for a method and apparatus for enhancing the overall picture contrast of video signals to provide better subjective quality, which also overcomes the shortcomings associated with over-correction and complexity using existing techniques.